Many different types of wastewater streams are treated by an evaporation process. Generally a feedwater is directed into an evaporator and the evaporator heats the feedwater, resulting in the production of steam and a concentrate. The steam condenses and forms a distillate that can be used for a variety of purposes. It is typical in evaporation processes to preheat the evaporator feedwater, deaerate the evaporator feedwater, and cool the distillate for storage or use. It is known to employ an indirect contact heat exchanger to transfer heat from the distillate to the evaporator feedwater. The heat recovered minimizes makeup heat duty required for the evaporator and cooling duty to sufficiently cool the distillate for storage or use.
Some evaporator feedwaters contain components such as calcium carbonate that precipitate when heated. One example of this would be produced water generated from oil and gas production. Using a heat exchanger in these cases is problematic. Scale deposits on the heat transfer surfaces and limits heat transfer, or in some cases, can actually plug the heat exchanger. Heat exchanger fouling requires additional cooling duty to sufficiently cool the distillate and additional makeup steam to compensate for the additional preheat load for the evaporator feedwater. In addition, the heat exchanger requires frequent cleaning which results in significant cost for cleaning chemicals and maintenance to keep them operative.
For evaporator feedwaters that include aggressive scaling components, the heat exchanger can scale or foul more rapidly than it can be cleaned. This makes employing a heat exchanger in such cases impractical. In this situation, makeup steam is needed to supply all the preheat duty for the evaporation system. This may be an impractical option depending on the incoming evaporator feedwater temperature.
Therefore, there has been and continues to be a need for an efficient system and process for preheating evaporator feedwater and cooling the distillate produced by an evaporator when the evaporator feedwater includes scaling species that make it impractical to employ an indirect heat exchanger.